The present invention relates to an optical fiber array as a component of an optical switch, an isolator, the input/output portion of an optical connection device, a semiconductor laser, an optical coupling component between a photodiode and an optical fiber, or a multicore optical connector.
With an abrupt increase in data traffic, a strong demand has arisen for an increase in the capacity of a trunk network. In this trunk network, a large-capacity optical network using a WDM (Wavelength Division Multiplexing) technique has already been introduced for data transmission equipment. However, the following scheme is used for a node portion. First, an optical signal is temporarily converted into an electrical signal, and paths are switched by a switch using a conventional electric circuit. Then, the signal is converted into an optical signal again to be returned to the data transmission equipment.
It is pointed out that such a device for converting optical and electrical signals will greatly increase in cost and power consumption with an improvement in signal bandwidth (see non-patent reference 1: A. S. Morris III, “In search of transparent networks”, IEEE Spectrum, pp. 47–51 (October 2001)). For this reason, studies have been made on the use of an optical switch designed to switch an optical signal without any optical and electrical signal convertor. A free-space type optical switch is, in particular, small, which uses a light beam for connection (optical connection) inside a switch or connection between switches without using any optical waveguide medium as wiring inside the optical switch, and hence the practical application of the optical switch to a switch portion of a large-scale network router has been studied.
FIG. 7 shows such a conventional free-space type optical switch (see non-patent reference 2: D. T. Neilson et al., “Fully provisioned 112×112 micro-mechanical optical cross connect with 35.8 Tb/s demonstrated capacity”, OFC2000. paper-PD12–1, (2000)).
An optical switch 110 is comprised of an optical fiber array 111, microlens array 112, micro-tilt mirror array 113, and stationary mirror 114. The optical fiber array 111 is designed such that optical fibers are two- or one-dimensionally aligned/arranged at predetermined intervals by using a fiber aligning member. The microlens array 112 is designed such that microlenses are two- or one-dimensionally aligned/arranged at predetermined intervals like the optical fiber array.
The micro-tilt mirror array 113 is designed such that a plurality of micro-tilt mirrors as active elements which are formed on a semiconductor wafer by using a micromachining technique are one- or two-dimensionally arrayed. An inclination angle θ of the mirror surface of each micro-tilt mirror can be dynamically changed. For the sake of simplicity, FIG. 7 shows each component as a one-dimensional array.
In this conventional free-space type optical switch 110, an optical signal 100 emerging from each optical fiber of the optical fiber array 111 is converted into collimated light by a corresponding microlens of the microlens array 112 and reflected by a corresponding micro-tilt mirror of the micro-tilt mirror array 113. The light is then reflected by the stationary mirror 114 and reflected again by a micro-tilt mirror of the micro-tilt mirror array 113. The reflected light is finally focused on an optical fiber of the optical fiber array 111 via a microlens of the microlens array 112.
In the optical switch 110 having the above arrangement, by adjusting the inclination angle θ of the micro-tilt mirror of the micro-tilt mirror array 113, the traveling direction of the optical signal 100 is switched to guide the optical signal 100 to the target optical fiber of the optical fiber array 111. This optical system constituted by the optical fiber and microlens, which is used for conversion or focusing for the optical fiber and a light beam, is generally called an optical collimator.
In the above optical switch 110, in addition to reflection losses at refractive-index boundaries between each optical component, the connection loss between input and output optical fibers dominantly includes the coupling loss between a light beam and the output optical fiber due to the inclination of the optical axis of the light beam which is caused by the beam-position displacement between the optical fiber and the microlens constituting an optical collimator, a clipping loss from a lens aperture, and a reflection loss at the refractive-index boundaries. The light beam that has undergone the optical axis inclination due to the beam-position displacement causes crosstalk between adjacent channels, resulting in a deterioration in optical channel quality.
In a two-dimensional collimator array, in particular, the amount of optical axis displacement between each optical fiber and a lens is greatly influenced by the optical fiber displacement in an optical fiber array. For this reason, in a two-dimensional collimator array, an improvement in array manufacturing precision is strongly required. Note that an apparatus using this optical collimator lens is not limited to an optical switch and is equally applied to an optical isolator using a light beam for connection and an optical interconnection apparatus. This array is also applied to a coupling portion between a semiconductor laser or photodiode and an optical fiber.
FIG. 8 shows a conventional two-dimensional optical fiber array used for a free-space type optical switch.
As shown in FIG. 8, a two-dimensional fiber array 120 is formed as follows. Optical fibers 121 are respectively inserted into V-groove portions of V-groove substrates 122 and aligned. The optical fibers 121 are temporarily fixed by fiber press plates 123 and fixed with an adhesive filled in air gaps between these components. The V-groove substrates 122 are stacked and boded with an adhesive. As the V-groove substrate 122, for example, a ceramic, glass, or silicon substrate in which V-groove portions are formed by using a high-precision machining technique is widely used. With this structure, the optical fiber displacement in the horizontal direction with respect to the substrate surface can be suppressed to 1 μm or less.
FIG. 9 shows the schematic structure of another conventional two-dimensional optical fiber array (MT type optical connector ferrule).
As shown in FIG. 9, a two-dimensional optical fiber array 130 has optical fibers 131 respectively inserted into alignment guide holes 132a of a ferrule 132 and fixed with an adhesive injected through an adhesive filling hole 132b. A polymer thermoplastic material (thermoplastic resin) exhibiting a small deformation amount at the time of thermal shrinking or after molding is used for the ferrule 132. The ferrule 132 is manufactured by a transfer-plastic molding technique of injecting the heated material into a mold and molding it by cooling. In general, plastic molding techniques represented by the transfer-plastic molding technique are suitable for mass production and allow high-precision optical fibers to be manufactured at a low cost.
In the two-dimensional fiber array 120 shown in FIG. 8, however, the following problems arise.                (1) First of all, when the V-groove substrates 122 are to be stacked on each other, the substrates are bonded after the substrates are positioned in the horizontal direction with respect to the substrates. The influences of the shrinking of an adhesive must be taken into consideration, and it is difficult to control the positions of the substrates in consideration of the shrinking. In the two-dimensional fiber array 120, therefore, an improvement in the positioning accuracy of the optical fibers 121 in the stacking direction of the substrates (the vertical direction with respect to the substrate surface) is limited.        (2) In addition, as the number of optical fibers 121 increases in the horizontal direction of the V-groove substrates 122 with an increase in scale, displacements of the optical fibers 121 are likely to occur due to the warpage of the V-groove substrates 122. As the number of V-groove substrates 122 stacked increases, displacements of the optical fibers 121 are likely to occur due to variations in thickness of the V-groove substrates 122 and the thickness of the adhesive. Even if the V-groove substrates 122 are formed with high dimensional precision, the overall optical fiber displacement increases due to a variation in the thickness of the adhesive.        (3) In general, each V-groove substrate 122 is manufactured by forming V-groove portions in, for example, a semiconductor, glass, or ceramic substrate by a high-precision machining technique or etching process. However, a manufacturing method using such a high-precision machining technique or process is not suitable for mass production, and hence it is difficult to reduce the manufacturing cost.        (4) Since the outer diameter of an optical fiber is as small as 125 μm, the optical fiber is very difficult to handle. Since such optical fibers that are difficult to handle are used, it is difficult to reduce the assembly cost for the two-dimensional fiber array 120 shown in FIG. 8.        
In the two-dimensional optical fiber array 130 shown in FIG. 9, the following problems arise.
In general, anti-reflection coating films made of dielectric-multilayer films are formed on the refractive-index boundaries between air and optical fibers and lenses constituting optical collimators in order to eliminate the influences of reflection. In many cases, such anti-reflection coating films are formed by vapor deposition. In vapor deposition, in the process of forming films, objects on which the films are to be formed are left in a high-temperature environment of several hundred ° C. or higher.
When the ferrule 132 shown in FIG. 9 is to be used, the plurality of optical fibers 131 are inserted into the guide holes 132a and held by the ferrule 132. In this state, the light incident/exit end faces of the respective optical fibers 131 are polished and anti-reflection coating films are formed on the polished light incident/exit end faces. However, the glass-transition temperature of the thermoplastic resin used for the ferrule 132 is near 180 to 200° C., and hence it is technically difficult to form an anti-reflection coating film by vapor deposition in the presence of the ferrule 132.